SEO, or Search Engine Optimization is a fancy term used to get more traffic to websites from natural and free searches that are made at major search engines like Google, Yahoo and Bing, whom act as the librarians of the internet. The systems of these search engines collect information about every page on the internet in order to help people find exactly what they are searching for. Every search engine makes use of an algorithm to change all that information into useful results.
If you are an owner of a website, these search results will matter to you. More people will find you if you have higher rankings. The key to achieve this is to make sure your website has the elements that search engines need for their algorithms and this is what is referred to as SEO.